1. Field of the Invention
The invention relates to a multi-stage process for the continuous preparation of melt-processable polyurethanes with improved processing characteristics in a twin screw extruder with special temperature control.
2. Description of the Prior Art
Thermoplastic polyurethane elastomers (TPU) are by no means new. They are of industrial importance due to the combination of high-quality mechanical properties with the well known advantages of low-cost melt processability. A wide variety of mechanical properties may be obtained by using different chemical structural components. A review of TPU, their properties and applications is given, e.g., in Kunststoffe 68 (1978) 819 or Kautschuk, Guinmi, Kunststoffe 35 (1982), 568.
TPU are composed of linear polyols, mostly polyester- or polyether-polyols, organic diisocyanates and short-chain diols (chain extenders). In addition, catalysts may be added to accelerate the formation reaction. The structural components may be varied in relatively wide molar ratios in order to adjust the properties. Molar ratios of polyols to chain extenders of 1:1 to 1:12 have proved suitable. Products in the region of 70 Shore A to 75 Shore D are thereby obtained.
The TPU may be prepared continuously or batchwise. Industrial preparation in extruders is by no means new (DE-OS 1 964 834). The structural components are metered individually or in the premixed state into the extruder and the reaction is carried out in the extruder at temperatures between 90 and 220.degree. C. which are preset by way of the extruder barrel. A disadvantage of this process is that the homogeneity and mould release behaviour of the TPU prepared in this way are not sufficient for all fields of application.
According to DE-OS 2 059 570, all the reaction components are fed simultaneously to an intensively mixing and kneading twin screw extruder. The machine is subdivided into a feed zone, a mixing and reaction zone, and a discharge zone. As a result of a barrel temperature profile which rises in a linear manner from the feed zone (30-127.degree. C.) to the discharge zone (177-249.degree. C.), the uniform viscosity required is obtained throughout the zones. The relatively low temperatures of 130 to 170.degree. C. in the reaction zone lead partly to rigid segment deposits. In view of the high final temperature in the discharge zone, the viscosity obtained is not sufficient for thorough mixing, despite the mixing elements, so that nodule-free product cannot be obtained. Moreover, products which are difficult to demould are obtained at these high final temperatures.
A slight improvement in the TPU homogeneity is obtained by a process described in DE-OS 2 610 980. In this case, the starting products are preheated to 180 to 250.degree. C. The barrel temperatures of the extruder are adjusted to a temperature profile which falls from the feed zone (180 to 250.degree. C.) to the discharge (165 to 200.degree. C.). In this way, solid deposits in the reaction part of the extruder are avoided. At the end of the screw, the product is prevented from becoming overheated by the reduced temperature and is extruded more easily at higher viscosities. A disadvantage of this process, however, is the greater dependence on the reactivity of the raw materials. With the usual variations in reactivity of the industrially available monomers, the TPU formation reaction starts so quickly at these high initial temperatures that even intensive monomer mixing cannot guarantee that a homogeneous mixture of reactants is present before the reaction starts. The inadequate mixing then leads to inhomogeneous products.
In order to improve the TPU homogeneity, the use of special conveying and mixing/kneading elements in the reaction extruder was also proposed in DE-OS 23 02 564. According to EP-A 708 124, these are distributed over four different zones in the extruder. The temperature and reaction control and catalyst metering must, however, be adjusted precisely to the screw geometry as a function of the raw material reactivity so that the critical reaction phase occurs exactly at the place where the kneading elements of the extruder are situated. The preparation of different product types is also difficult with a single screw geometry.
EP-A 571 830 describes how, in a simple batch process, by reacting polyol with a partial quantity of the diisocyanate, mixing in the remaining diisocyanate and subsequent chain extension, a TPU is obtained with a markedly higher recrystallisation temperature compared with the standard process, which permits more rapid mould release. The products thus obtained, however, result in films containing specks due to the production process, and are therefore unsuitable for processing by extrusion.
It has now been found that, by means of a new production process with a single screw geometry, various TPU products may be prepared with improved mould release characteristics and a high degree of homogeneity, particularly for the extrusion sector.